I use 1080x1920 resolution 59.94fps .MTS AVCHD footage AND .mxf files rendered out of After Effects with the same resolution and frame rate.

I also import 1080 x 1920 Photoshop documents (through dynamic link).

The final product is usually less than 10 minutes long.

I export using H.264 with the same settings as the input footage with a target bitrate of 25mbps and a maximum bitrate of 30mbps.

This is for YouTube videos.

Now, to the main question:

I'm not sure if there's any type of bottleneck in this system that would be causing Premiere to usually render around 70%, but I'm just wondering; if I were to replace my 1TB 850 Pro with, say, a 512GB or 1TB 950 Pro NVMe drive, would there by any noticeable performance increase whatsoever (whether it be in terms of playback dropping fewer frames or CPU utilization during export or anything else)? I don't really understand the advantage NVMe drives realistically offer over a SATA III SSD like the 850 Pro.

the 950 Pro is BI-DIRECTIONAL....meaning it will read AND write data AT THE SAME TIME, unlike SATA drives which can only read OR write...they cannot do both operations at the same time. This means that latency is LESS, and combined with the HUGE speed difference of 2GB/sec read speed vs. 550MB/sec on the 850 Pro, the overall performance SHOULD be better, especially if using multiple video tracks at such a high frame rate.

The CPU running at 70% could be a symtom of the GPU being "saturated " to 100% GPU usage, due to effects and the high frame rate.......the CPU might be "waiting" on the overtaxed GPU. This is easily seen if you use a free program like MSI Afterburner,or, similar, to monitor GPU usage and performance while editing, rendering previews ,or, exporting.

Adding 1 or even TWO more 850 Pro SSDs and creating a RAID0 would be a similar idea to adding a 950 Pro,but, would give you a much greater capacity, while tripling your drive speed. The 950 Pro only has a 500GB capacity and would require you to "offload" completed projects, source materials and exports to an archive drive. Since the 850 Pros are so reliable, a RAID 0 of 3 of them would provide a read and write speed of about 1.5 GB/sec. and a capacity of 3TB without much worry of drive failure. You could always back that RAID 0 up using an "enterprise level" HDD which is 3 or 4 TB in size. There are a few good ones that will do over 200MB/sec read and write with a 128 MB cache.

Using two separate video cards only helps in some operations, like exporting , ( according to Bill Gehrke who has tested this ).so, you SHOULD be exporting faster with the two cards,but, may be a single GPU that is more powerful could do better, ( like a 980ti, or, Titan ).

Many users like Bill are overclocking that CPU to 4.5 Ghz to get the best performance...are you doing this ?

You may want to visit Bill's website, PPBM7.com, to test your machine with his PPro benchmark test. it will identify any "bottlenecks' and will allow you to compare your machine with others.